Backpack frame



FIG. 1 is a front, side perspective view of a backpack frame, showing the new design;

FIG. 2 is left side view thereof;

FIG. 3 is a right side view thereof;

FIG. 4 is a top view thereof;

FIG. 5 is bottom view thereof;

FIG. 6 is a front view thereof; and,

FIG. 7 is a rear view thereof.

All broken lines represent portions of the backpack frame that form no part of the claimed design. 

CLAIM The ornamental design for a backpack frame, as shown and described. 